Phased Array Radar Antennas are described in chapter 7 and advanced ECCM for radars is described in chapter 9 of The Radar Handbook, edited by Merrill Skolnik, published by McGraw-Hill Publishing Co. (2d ed. 1990), and herein incorporated by reference. As described in Monopulse Principles and Techniques, written by S. M. Sherman, published by Artech House (1984), and in Monopulse Radar, written by A. I. Leonov and K. I. Fomichev, and published by Artech House, Inc. (1988), both of which are herein incorporated by reference, monopulse processing for a planar antenna array for radar typically involves the synthesis of sum and delta beams, as is well-known for a rectangular antenna aperture. For a rectangular aperture the beams may also be separable in azimuth and elevation, which is desirable for advanced electronic counter-countermeasures (ECCM) while preserving the monopulse ratio, as described in "Combining Sidelobe Canceller and Mainlobe Canceller for Adaptive Monopulse Radar Processing," patent application Ser. No. 07/807,548, filed Dec. 16, 1991, by Yu et al., "Adaptive Digital Beamforming Architecture and Algorithm for Nulling Mainlobe and Multiple Sidelobe Radar Jammers While Preserving Monopulse Ratio Angle Estimation Accuracy," patent application Ser. No. 07/807,546, filed Dec. 16, 1991, by Yu et al., and "Simultaneous Sidelobe and Mainlobe Radar Jamming Canceller for Adaptive Monopulse Processing," patent application Ser. No. 07/912,398, filed Jul. 13, 1992, by Yu et al., all assigned to the assignee of the present invention and herein incorporated by reference. Presently, various antenna apertures other than rectangular apertures, such as circular antenna apertures, are available for use in radar systems and offer improved performance over rectangular apertures, although such apertures are susceptible to jamming. Examples of such nonrectangular apertures are described in The Antenna Handbook, edited by Y. T. Lo and S. W. Lee, and published by Van Nostrand Reinhold Co. (1988). A need exists for an antenna aperture that is less susceptible to advanced ECCM and also provides the improved performance available from nonrectangular apertures.